If a project's members choose to license their code under the GPL, those members should not be held responsible for the decisions of others not under their control to reject GPL code. On the other hand, those who reject GPL code do so because they don't want others to re-use their own derivatives of that code, so it is in fact those that reject the GPL and other copyleft licenses, not those that proliferate them, who are discouraging code re-use.
It is true that not every project entity that chooses the GPL for its output does so for the purposes of encouraging code re-use. Some projects make their work available under both GPL and proprietary licenses in order to enable the commercial entities financially backing the project to sell proprietary licenses to the code. Both such asynchronous licensing situations and permissive licensing situations have drawbacks compared to a pure GPL approach: for permissively licensed code, the rights of code re-use are not protected to the same extent that they are with the GPL, while for dual-licensed code, one or several entities is placed in a privileged position relative to that of other stakeholders.
However, since the output of such dually-licensed projects is available under a GPL license at no charge, the only potential adopters of the code who are directly affected by the availability of a proprietary license to the code are those that had already made the decision to reject the GPL. Those that were seeking to use GPL code in the first place can still do so and are not directly negatively impacted by the availability of proprietary licensing options.